Horizontal color plane registration correction

ABSTRACT

A method for detecting and correcting misregistration between a plurality of separations printed by a printing apparatus, comprising: producing a calibration print output on a substrate, the calibration print output including at least one calibration pattern, wherein the at least one calibration pattern translates horizontal misregistration into a detectable indicator of misregistration in a process direction; detecting misregistration; and, performing a fine adjustment of the horizontal position of at least one of the separations using the at least one calibration pattern.

FIELD OF THE INVENTION

The present invention relates in general to printing and particularly todetecting and/or correcting misregistration in printing.

BACKGROUND OF THE INVENTION

When multicolor information is to be imaged or written, a final compoundcolor is obtained, in general, by superimposing print separations. Eachprint separation has a different basic color, and the color separationprints are coordinated with and aligned relative to each other. Ingeneral, a plurality of dots or patches, each of different basic colors,are printed in a same locality so as to be aligned with or superimposedon each other. Such superposition of print separations gives theimpression of a full color image having colors that may be differentfrom the basic colors.

Normally three or four separations are used, each with a basic (process)color, (or optionally, black) in order to obtain a final compound color.In some cases additional color separations are also used. The finalcompound image is obtained by finely adjusting, through alignment of thesystem, the position of each separation, to accurately overlay theseparation prints. The alignment process and the alignment itself arecalled registration.

When the separations are printed slightly out of registration, theappearance of an image is slightly impaired. However, if the separationsare more than slightly out of registration, the effect will bedisturbing to an observer. In particular, the individual edges ofobjects formed by each one of the separations will separate and thequality of the final multicolor image will be greatly impaired.

WO 00/43206 to Indigo, N. V., the disclosure of which is hereinincorporated by reference, describes an apparatus and method forcorrecting print separation registration in a printing apparatus orphotocopier.

SUMMARY OF THE INVENTION

An aspect of some exemplary embodiments of the invention relates toproviding a method for detection and correction of horizontalmisregistration between a plurality of print separations, whichtranslates horizontal misregistration into an indication ofmisregistration in a vertical, process direction. Optionally, the methodis capable of detection and correction of horizontal misregistrationusing one calibration pattern. In an embodiment of the invention, themethod is used for correcting horizontal misregistration inelectrostatic printing apparatus.

In an exemplary embodiment of the invention, a calibration print outputis produced by a print engine for registration/misregistration analysis.The calibration print output is used to detect and correct anymisregistration of the plurality of separations and/or to detect if theplurality of separations is registered. Optionally, a plurality ofseparations is considered registered if misregistration is within aprescribed acceptable limit.

The calibration print output is finely adjusted by analyzing the atleast one calibration pattern with respect to an associated offsetmarker on the output, in accordance with an exemplary embodiment of theinvention. As indicated above, the at least one calibration pattern isconfigured such that horizontal misregistration is translated into aprocess direction (i.e. vertically) detectable indicator. In anembodiment of the invention, analysis of the at least one calibrationpattern is performed by a controller.

In an embodiment of the invention, the plurality of print separationscomprise lines that form an interference pattern forming, for example, aMoiré pattern. In this embodiment, the Moiré pattern comprises a one ormore dark and/or light bands whose position in the process directionreflects the quality of horizontal (cross-scan) registration.Optionally, the indicator is a light band. The associated offset marker,against which the detectable indicator is measured, is also adapted tobe detectable by at least one sensor for translation of any horizontalmisregistration in the process direction. An analysis of the calibrationpattern against the offset markers, manually and/or automatically, andinputting the data derived from the analysis into the controllerindicates to the controller what adjustments need to be made in theoperation of the printing apparatus in order to provide printingapparatus operation within the prescribed acceptable limits. Optionally,the calibration pattern is comprised of more than one color. Optionally,the calibration pattern is comprised of a plurality of separations.Optionally, a rough adjustment is performed prior to the fineadjustment. Optionally, the dark band of the Moiré pattern is used todetermine the horizontal alignment.

Optionally, the horizontal alignment is adjusted by adjusting the timingof the writing of the horizontal lines for one or both separations. Insome embodiments of the invention, the horizontal offset and/or scalingof at least one separation is adjusted to implement at least a partialcorrection of misregistration. In an embodiment of the invention, thefrequency of the pixel clock may be adjusted. Optionally, the pixelclock is adjusted dynamically, for example in response to commandsreceived from the controller.

In an embodiment of the invention, the process of producing acalibration print output, rough adjusting the output and/or fineadjusting the output is repeated using at least one incremental step inorder to bring printing apparatus operation within the prescribedacceptable limits, as necessary.

An aspect of some exemplary embodiments of the invention relates toproviding a printing apparatus including a controller programmed toconduct a method for detection and correction of horizontalmisregistration between various print separations using one calibrationpattern. In some embodiments of the invention, the same calibrationpattern is printed in a plurality of locations on a calibration printoutput for misregistration analysis. Optionally, the controllerautomatically performs the method. In some exemplary embodiments of theinvention, the printing apparatus is provided with at least one sensorin operative communication with the controller for measuring at leastone indicator of horizontal misregistration (and/or registration if itis determined there is no misregistration within prescribed acceptablelimits). Optionally, an indicator is at least two rough adjustmentlines. Optionally, an indicator is a calibration pattern andcorresponding offset markers. In some exemplary embodiments of theinvention, the controller is operatively connected to a user inputdevice for receiving manually detected data regarding horizontalmisregistration. Based on the data received from the at least one sensorand/or the user, controller adjusts operation of the printing apparatusto provide an output closer to, or within, the prescribed acceptablelimits. Optionally, the controller repeats its program, each repetitioncomprising at least one incremental step towards bringing printingapparatus operation within the prescribed acceptable limits (e.g. inscenarios where the printing apparatus is operating outside of thoselimits). If the vertical indication is linear with respect to thehorizontal misregistration, then it may not be necessary to repeat thecorrection.

There is thus provided in accordance with an embodiment of theinvention, a method for detecting and correcting misregistration betweena plurality of separations printed by a printing apparatus, comprising:producing a calibration print output on a substrate, the calibrationprint output including at least one calibration pattern, wherein the atleast one calibration pattern translates horizontal misregistration intoa detectable indicator of misregistration in a process direction;detecting misregistration; and, performing a fine adjustment of thehorizontal position of at least one of the separations using the atleast one calibration pattern. In some embodiments of the invention, aplurality of offset markers are associated with the at least onecalibration pattern. In some embodiments of the invention, the fineadjustment is performed using the at least one calibration pattern andthe associated offset markers. In an embodiment of the invention, themethod further comprises producing the calibration print output with aplurality of rough adjustment lines in addition to the at least onecalibration pattern. Optionally, the method further comprises correctingmisregistration by performing a rough adjustment using the plurality ofrough adjustment lines prior to performing the fine adjustment.

There is further provided in accordance with an embodiment of theinvention, a method for detecting misregistration between a plurality ofseparations printed by a printing apparatus, comprising: (a1) producinga calibration print output on a substrate, the calibration print outputincluding at least one calibration pattern with a plurality ofcorresponding offset markers, wherein the at least one calibrationpattern translates horizontal misregistration into a detectableindicator of misregistration in a process direction. In an embodiment ofthe invention, the method further comprises: (a2) producing thecalibration print output with a plurality of rough adjustment lines inaddition to the at least one calibration pattern. In an embodiment ofthe invention, the method further comprises: correcting misregistrationby performing at least one of the following: (b) a rough adjustmentusing the plurality of rough adjustment lines; and, (c) a fineadjustment using the at least one calibration pattern and thecorresponding offset markers. In an embodiment of the invention, themethod further comprises: correcting misregistration by performing: (b)a fine adjustment of the horizontal position of at least one of theseparations using the at least one calibration pattern and thecorresponding offset markers. Optionally, the calibration pattern isproduced from a plurality of sub-patterns. Optionally, each of theplurality of sub-patterns is associated with one of the plurality ofseparations. Optionally, at least one of the plurality of sub-patternsis produced in a different color than the rest of the plurality ofsub-patterns. Optionally, at least one of the plurality of sub-patternsis produced in the same color as another of the plurality ofsub-patterns. In some embodiments of the invention, performing a fineadjustment includes controlling operation of at least one laser of theprinting apparatus to cause a light band in the calibration pattern tobe centered in relation to the corresponding offset markers. Optionally,the calibration print output is produced with a calibration pattern onthe back of the substrate. Optionally, each of the plurality of roughadjustment lines is associated with one of the plurality of separations.Optionally, at least one of the plurality of rough adjustment lines isproduced in a different color than the rest of the plurality of roughadjustment lines. In some embodiments of the invention, performing arough adjustment includes controlling operation of the printingapparatus to cause the plurality of rough adjustment lines to at leasttouch.

There is further provided in accordance with an embodiment of theinvention, a printing apparatus for detection and correction ofhorizontal misregistration between a plurality of print separations,comprising: a latent image forming device adapted for forming aplurality of latent images of the plurality of print separations,developing the latent image, and printing the latent image on asubstrate; a controller in communication with at least the latent imageforming device adapted to control the latent image forming device in theforming of a calibration pattern, which translates horizontalmisregistration into a detectable indicator of misregistration in aprocess direction, sufficient for detection and correction of horizontalmisregistration between the plurality of print separations on acalibration print output; and, wherein upon receipt by controller of anindication of horizontal misregistration, controller controls the latentimage forming device to form the plurality of latent images in at leastan incremental step towards correcting the horizontal misregistration.In an embodiment of the invention, the apparatus further comprises auser input device including a display which prompts a user to inputdetected horizontal misregistration in communication with thecontroller. In an embodiment of the invention, the apparatus furthercomprises at least one sensor adapted to sense at least one indicator ofhorizontal misregistration on the calibration print output andcommunicate the sensed misregistration to the controller. Optionally, atleast one indicator of horizontal misregistration is a calibrationpattern comprised of at least two sub-patterns which when overlappedform a light band within the calibration pattern. Optionally, thecalibration pattern includes a plurality of corresponding offsetmarkers. Optionally, the sensor is positioned to sense the calibrationpattern printed on the back of the substrate. Optionally, the sensor ispositioned to view more than one calibration pattern is printed on thecalibration print output. In some embodiments of the invention, at leastone of the plurality of separations is printed in a different color thanthe rest of the separations. In some embodiments of the invention, atleast one of the plurality of separations is printed in the same coloras another of the separations.

BRIEF DESCRIPTION OF FIGURES

Exemplary non-limiting embodiments of the invention are described in thefollowing description, read with reference to the figures attachedhereto. In the figures, identical and similar structures, elements orparts thereof that appear in more than one figure are generally labeledwith the same or similar references in the figures in which they appear.Dimensions of components and features shown in the figures are chosenprimarily for convenience and clarity of presentation and are notnecessarily to scale. In the attached figures:

FIG. 1 is a schematic of a printing apparatus suitable for detection andcorrection of misregistration between various separations, in accordancewith an exemplary embodiment of the invention;

FIG. 2 is a calibration print output, in accordance with an exemplaryembodiment of the invention;

FIG. 3A depicts a detailed view of a calibration pattern and offsetmarkers in a registered condition, in accordance with an exemplaryembodiment of the invention;

FIG. 3B depicts a detailed view of a calibration pattern and offsetmarkers in a misregistered condition; and,

FIG. 4 is a flow chart depicting a method of measuring and correctingmisregistration between various separations, in accordance with anexemplary embodiment of the invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIG. 1 shows a portion of an electrostatic printing apparatus 100suitable for detection and/or correction of separation misregistration,in accordance with a preferred embodiment of the invention. Themethodology of the image formation using printing apparatus 100 canaccomplished using any of a wide variety of different available powderor liquid toner systems, for example the HP Indigo® Press 5000,available from Hewlett-Packard. In general, the present invention doesnot appear to be tied to any particular system of printing apparatus,although the cause and severity of misregistration problems may dependon the imaging method and particular/or imaging system used. In thefollowing discussion, the printing apparatus is described in rathergeneral terms to emphasize the general application of the invention.

In an exemplary embodiment of the invention, a photoreceptor 102 iselectrified by a corotron, scorotron or other electrifying means 104. Ascanning laser 106 beam or beams impinge on photoreceptor 102 and form alatent image of a particular separation thereon. A dispenser of liquidtoner 108 which may be, for example, a binary ink developer (“BID”)cartridge, as described in one or more of U.S. Pat. No. 5,596,396 toLanda et al., and U.S. Pat. No. 5,610,694, to Lior et al., thedisclosures of which are incorporated by reference. Optionally, aplurality of BID cartridges is used, each containing a different colortoner. Alternatively, the dispenser of liquid toner is any one of thefollowing: a spray dispenser, a series of spray dispensers or a seriesof slit dispensers, as known in the art, supply a liquid toner of acolor corresponding to the separation. Optionally, the toner is a powdertoner, applied in any manner known in the art.

The latent image is developed by the toner to form a visible image onphotoreceptor 102. In some embodiments of the invention, a squeegeeroller 112 compresses the image and removes excess liquid therefrom,prior to the transfer of the image to an intermediate transfer member(“ITM”) 114. The image is then transferred to a substrate 118 at a nipbetween ITM 114 and an impression roller 116. After transfer of theimage to ITM 114, residual toner and charge on photoreceptor 102 areoptionally removed by discharge and cleaning apparatus 120 which may beany of the many types that are well known in the art.

Individual separations are written (by scanning laser 106), developedand transferred to substrate 118 in seriatim. Optionally, a plurality ofseparations (or all the separations) may be transferred to the ITM firstand then transferred as a group to the final substrate.

Unfortunately, the horizontal registration of the separations may not beperfect. Thus, in a calibration operational mode for printing apparatus100, the image created is a calibration print output (described in moredetail with respect to FIGS. 2-4), which is optionally used fordetection and/or correction of horizontal separation misregistration, inaccordance with an exemplary embodiment of the invention. In anexemplary embodiment of the invention, calibration for correctinghorizontal misregistration is conducted by printing apparatus 100 usingat least a controller 122.

Controller 122 is programmed with software to, inter alia, control thelaser to write the latent image for the separations. In addition,controller 122 receives data pertaining to separation misregistrationand makes corrections in the operation of printing apparatus 100 tocorrect the misregistration. For example, by directing scanning laser106 to alter its latent image formation on photoreceptor 102 such thatmisregistration will not take place, or will be reduced to within aprescribed acceptable limit. In some embodiments of the invention, thehorizontal offset and/or scaling of at least one separation is adjustedto implement at least a partial correction of misregistration.Optionally, the horizontal misregistration is corrected by adjusting thetiming of the writing of the horizontal lines by scanning laser 106, forat least one separation. Optionally, the timing is adjusted dynamically,for example in response to commands received from controller 122. In anembodiment where data pertaining to separation misregistration isprovided to controller 122 manually, for example by a user of printingapparatus 100, the calibration print output is produced by printingapparatus 100 and is examined by the user for perceived misregistrationproblems.

Printing apparatus 100 is optionally provided with a user input device126, for example including a keyboard, with which to enter user-detectedhorizontal misregistration data. Examples of what and/or how the userenters data are described below with respect to FIGS. 2-4. Optionally,user input device 126 is provided with a display for displayinginformation pertaining to printing apparatus 100 to user. In anexemplary embodiment of the invention, user input device 126 is inoperative communication with controller 122. In some embodiments of theinvention, the user of printing apparatus 100 is prompted by software toenter misregistration data. Optionally, controller 122 is programmedwith the software which is used to prompt the user to entermisregistration data. Optionally, the software which is used to promptthe user is the same software used to control the laser, as describedabove. It should be understood that user input device 126 is optionallyprovided to printing apparatus 100 without regard to whether datapertaining to separation misregistration is provided manually, forexample to allow the user to operate printing apparatus 100.

In some embodiments of the invention, data pertaining to separationmisregistration is provided to controller 122 automatically. In someexemplary embodiments of the invention, the print engine is providedwith at least one sensor 124, in operative communication with controller122, for measuring at least one indicator of horizontal misregistration(and/or registration if it is determined there is only slightmisregistration within prescribed acceptable limits). Optionally, anindicator is at least two rough adjustment lines (described below).Optionally, an indicator is a calibration pattern and a plurality ofoffset markers (described below). Optionally, the at least one sensor124 is a densitometer provided with an appropriate resolution forcarrying out the final adjustment described herein.

It should be noted that the printing apparatus configuration shown isonly used to illustrate an exemplary configuration for carrying out thepresent invention and that other configurations could be used to achievesubstantially the same effect (e.g. optionally, ITM 114 is not used).

Referring to FIG. 2, a calibration print output 200 is shown inaccordance with an exemplary embodiment of the invention. In anembodiment of the invention, calibration print output 200 is used totranslate horizontal misregistration into at least one detectableindicator in the process, or vertical, direction 210. Calibration printoutput 200 is comprised of a plurality of separations printed onto asubstrate, in some embodiments of the invention.

Calibration print output 200 is provided with at least one calibrationpattern 206 optionally with a plurality of offset markers 208, shown inthe calibration patterns located on the left side of calibration printoutput 200 in FIG. 2. Calibration pattern 206 and offset markers 208 aredescribed below. In an exemplary embodiment of the invention,calibration pattern 206 and the corresponding offset markers 208 areused for making fine adjustments in horizontal registration, for exampleas described with respect to FIG. 4. In an exemplary embodiment of theinvention, a plurality of calibration patterns 206 and correspondingoffset markers 208 are printed in various locations on calibration printoutput 200 to correct for misregistration in a plurality of places onsubstrate 118.

Optionally, a visually perceptible scale 212, shown in the right sidecalibration patterns of FIG. 2, is printed in calibration pattern 206for performing manual fine adjustments of misregistration.

In some exemplary embodiments of the invention, at least two roughadjustment lines 202, 204 are printed as a pair on calibration printoutput 200, with each rough adjustment line corresponding to aseparation. Optionally, rough adjustment lines are printed in more thanone location on calibration print output 200, for example on the leftand right sides, such as shown in FIG. 2. Optionally, each roughadjustment line of each pair is printed in a different color, assistingwith the identification of the individual separations and/ormisregistration errors. In an exemplary embodiment of the invention,rough adjustment lines 202, 204 are used for making rough adjustments inhorizontal registration, for example as described below and with respectto FIG. 4.

FIG. 3A, shows a detailed view of a calibration pattern 206 and offsetmarkers 208 in a registered condition, in accordance with an exemplaryembodiment of the invention. In some embodiments of the invention,calibration pattern 206 is comprised of at least two overlappingsub-patterns. Each sub-pattern is optionally associated with one of aplurality of separations used to produce calibration pattern 206. Insome exemplary embodiments of the invention, both separations areprinted in black, for maximum contrast. Since misregistration isgenerally not a function of the color, black is used since it gives ahigh contrast. Optionally, each separation is printed in a differentcolor. For example, the first separation is optionally printed inmagenta, whereas a different separation is optionally printed in black.Other typical colors for print separations include cyan and/or yellow,although colors and/or more than two separations are not required forthe instant misregistration correction method. It should be understoodthat only a small portion of calibration pattern 206 is shown, but in anexemplary embodiment of the invention, calibration pattern 206substantially occupies the box shown in FIGS. 2-3A-B.

In an embodiment of the invention, each sub-pattern comprises an arrayof lines, generally running in the process direction and closely spacedin the horizontal direction. The lines in each sub-pattern form an anglewith the process direction which is different from the angle for theother sub-pattern. Thus, the lines in the subsets cross at a fixedspacing in the process direction, given by s/(sin α), where s is thespacing between the lines and α is the angle at which the lines in thetwo sub-patterns cross. For small α, this reduces to s/α (α in radians)

Using this formula, a line spacing of 1 mm and an angle of 4 degrees(=4π/180 radians), the distance between crossings is about 14 mm. Moreimportantly, when one of the sub-patterns is moved 100 micrometers (0.1mm) horizontally with respect to the other, the crossings move about 1.4mm in the process direction. This vertical offset is linear with thehorizontal offset and measurements of the relatively large verticaloffset allows for measurement, with substantial accuracy, of very smallhorizontal offsets.

It is understood that halfway between the crossings, each set of lineswill be exactly between the lines. Thus, at the crossings a maximumunprinted area is produced and between the crossings a minimum unprintedarea is produced. If the lines are half the spacing, then the contrastis a maximum. This results in alternating light and dark bands in theprinted result.

These values of angle and spacing and line width are just an example andother values can be used if different sensitivity or contrast isdesired.

In an exemplary embodiment of the invention, a first sub-pattern 302 isprinted during the first separation using a given color. Optionally, thecolor is black or magenta. A second sub-pattern 304 is printed as asubsequent separation, in accordance with an embodiment of theinvention. Optionally, the subsequent separation is the final separationof the image being printed. Optionally, second sub-pattern 304 isprinted in a black color. Optionally, the lines in each sub-pattern arebetween 0.25 and 0.75 microns apart. Optionally, the lines in eachsub-pattern are approximately 0.5 microns apart. In some embodiments ofthe invention, for example in manual embodiments, line spacing isselected for ease of perceptibility to the human eye. In someembodiments of the invention, line spacing is selected which results ina vertical offset that is readable by automatic sensors.

In an exemplary embodiment of the invention, when calibration pattern206 comprised of the at least two overlapping sub-patterns 302, 304 isobserved and magenta and black are used as the print colors, a lightband 306 appears where the lines of the two sub-patterns 302, 304 touchor overlap. Where they do not touch or overlap, the calibration patternassumes a color similar to that of the magenta separation. It is notedthat if black and magenta are used then the pattern will have a reddishtint when the lines do not cross, which will disappear when they crossand the magenta is obscured by the black. Alternatively cyan or yellowcould be used.

Offset markers 208 are provided to calibration pattern 206 which aredetectable by a sensor, for example a densitometer. In an embodiment ofthe invention, offset markers 208 are arranged on calibration pattern206 in a manner that will convey to controller 122 the approximateposition of light band 306 on the calibration pattern 206. It should beunderstood that the sub-patterns 302, 304 are overlapped in a mannersuch that if first sub-pattern 302, and thus the first separation, isregistered with second sub-pattern, and thus the subsequent separation,then light band 306 is centered at the “0” (offset marker 308) on theoffset markers 208. In an exemplary embodiment of the invention,corresponding offset markers 208 are related to calibration pattern (forexample by printing it together with one of the separations) so thatslight horizontal misregistration, approx. 0.5 mm in some embodiments,of the sub-patterns 302, 304 causes light band 306 to move up or down inrelation to offset markers 208 (for example as shown in FIG. 3B), inprocess direction 210, by 30 mm. This movement by light band 306 inrelation to offset markers 208 is the process (vertical) directionmanifestation of horizontal misregistration, in accordance with someembodiments of the invention. It should be noted that the location ofoffset markers above and below the “0” marker are stored in controller122, such that when light band 306 is detected near a particular offsetmarker, the approximate distance and direction of misregistration isindicated, in accordance with an exemplary embodiment of the invention.

In some embodiments of the invention, the separations are configuredthat the white (or colorless) band is at the center of the pattern (asshown in FIG. 3A). Alternatively, the patterns are configured so thatthe dark (or reddish) band is at the center.

In addition to horizontal misregistration correction, verticalmisregistration can be corrected utilizing the patterns and method shownin WO 00/43206 and WO 2004/088969, the disclosures of which areincorporated herein by reference. Alternatively, other methods, as knownin the art, may be used to correct vertical misregistration. Inprinciple, vertical misregistration should be corrected first to avoidits effect on the measurement of horizontal registration. However, sincethe effect of horizontal misregistration on the band position is manytimes greater than that of the vertical misregistration, in practice,the order may not be important.

Since the horizontal alignment mechanism described above is verysensitive, it may be necessary to perform a coarse horizontal alignmentbefore utilizing the above described method. Any of the many methods ofproviding this coarse alignment can be used, for example, if twoparallel arrays of lines running in the process direction are provided(for example lines 202 and 204, then the density and possibly the colorof the image will be a function of the spacing between the lines. Thelines to be printed are designed so that they print one top of eachother when alignment is correct. When two different colors are used forthe separation (such as for example black and magenta) horizontalmisregistration will manifest itself in impartation of a reddish hue tothe area of the lines. The direction and approximate amount ofmisregistration can be determined from a magnified view of the print.

Another method of performing rough adjustment is to use the samemethodology as described with respect to FIG. 3 and adjusting thespacing and angles so that the sensitivity is convenient for measuringlarger misregistration.

Alternatively, in some embodiments of the invention, a simple visualobservation of a printed image may be sufficient to bring the image intorough registration.

FIG. 4 is a flow chart 400 depicting a method of measuring andcorrecting misregistration between various separations, in accordancewith an exemplary embodiment of the invention. It should be understoodthat that present methodology could be applied to any printing system orapparatus in which more than one separation is formed during printing.Calibration print output 200 is produced (402) by printing apparatus100, in an embodiment of the invention. Typically, production (402) ofcalibration print output 200 occurs by first using scanning laser 106 towrite sub-pattern 302 of a first of the separations used to comprisecalibration print output to form a latent image on photoreceptor 102.This sub-pattern 302 image is developed in one of the colors by elements108 and 110, as described above. The developed image is transferred tosubstrate 118. Next, a latent image corresponding to a second separationis written on photoreceptor 102. The latent image is then developed,preferably using a different color developer from that used to developthe first separation. This image is then transferred onto the image ofthe first separation on substrate 118. This results in the printedimages shown in FIGS. 2-3. It should be understood that in somepreferred embodiments of the invention, the images may be transferreddirectly to substrate 118 from photoreceptor 102, with ITM 114 omitted.Alternatively, both images may be transferred to ITM 114 before they aretransferred together to substrate 118. In this case, the measurementsmay be made directly on the ITM. In some exemplary embodiments of theinvention, more than two separations are used for forming calibrationprint output 200. For example, different calibration patterns areoptionally used for registering different combinations of separations.

In an exemplary embodiment of the invention, the produced (402)calibration print output 200 is used to detect and correctmisregistration between the separations. Optionally, a rough adjustmentperformed (404) on calibration print output 200 using rough adjustmentlines 202, 204, or by any other means known in the art. In someembodiments of the invention, a first rough adjustment line 202 isprinted in a different color than second rough adjustment line 204 sothat they can be easily discerned from one another. In some embodimentsof the invention, rough adjustment (404) is performed if the fineadjustment indicator, light band 306, is far removed from the “0” ofcorresponding offset markers 208 or is not present within calibrationpattern 206 due to being “off the scale”. Rough adjustment is optionallyperformed (404) if the rough adjustment lines 202, 204 do not touch oroverlap each other on calibration print output 200, in an exemplaryembodiment of the invention.

In some embodiments of the invention, a user of printing apparatus 100manually inputs the misregistration data into controller 122 via userinput device 126. In some embodiments of the invention, at least onesensor 124 automatically detects misregistration and communicates themisregistration data to controller 122. In a preferred embodiment of theinvention, at least one sensor 124 is placed near the surface ofcalibration output 200 to measure the position of light band 306 and acorresponding offset marker 208 in a calibration pattern 206. Othersensors, as known in the art, may be used to measure the position of themarkers 208, such that the controller can determine the relative offsetof the band with respect to the markers.

In an exemplary embodiment of the invention, misregistration dataincludes an indication of whether first rough adjustment line 202 is tothe left or right of second rough adjustment line using − or +. Inaddition, the instant methodology is intended to incrementally improvemisregistration errors, and therefore the indication of left or right isalso accompanied by a correction integer indicating how much left orright correction should be made by printing apparatus 100 in order tocause rough adjustment lines 202, 204 to touch or overlap. Optionally,the correction integer varies depending on the sensed or perceivedmisregistration error. Optionally, the correction integer is setdepending on how precisely printing apparatus 100 can be controlled.Optionally, the correction integer is changed from one production (402)of calibration print output 200 to the next production (402), forexample to make more fine adjustments.

In accordance with an exemplary methodology of the present invention, iffirst rough adjustment line 202 is to the left of second roughadjustment line 204, signaling that the first separation is horizontallymisregistered to the left of the subsequent separation, controller 122receives “−6” as misregistration data, where “−” indicatesmisregistration of the first separation to the left and the “6”indicating units, each unit comprised of 30 microns, of incrementalcorrection to be implemented by printing apparatus 100 for the nextprinting. Optionally, each unit is comprised of more or fewer micronsdepending on the application or adjustment accuracy desired. In anexemplary embodiment of the invention, should first rough adjustmentline 202 be to the right of second rough adjustment line 204, controller122 would receive “+6” as misregistration data. In an embodiment of theinvention, where the rough adjustment lines 202, 204 touch or overlap,no rough adjustment correction is to be made, thereby allowing for theinput of “0” into controller 122 for rough adjustment misregistrationdata. In an embodiment of the invention, production (402) of calibrationprint outputs 200 and rough adjustment is performed (404) is repeated(406) until the incremental corrections of misregistration produce acalibration print output 200 wherein rough adjustment lines 202, 204touch or overlap.

If a less sensitive version of the methodology of the fine adjustment isused for coarse adjustment, then this can be entered automatically, inthe same way as described below.

In an exemplary embodiment of the invention, a fine adjustment isperformed (408) after the optional rough adjustment (404). As describedabove, with respect to FIG. 3, calibration pattern 206 is analyzed todetermine the approximate center of light band 306 with respect tocorresponding offset markers 208. The value of the offset marker 208which correlates to the approximate center of light band 306 iscommunicated to controller 122, in some embodiments of the invention.Optionally, the value assigned to the offset marker 208 which correlatesto the approximate center of light band 306 is determined to within 0.5unit or 15 micron accuracy. In some embodiments of the invention, centerof light band 306 is associated with an offset marker which is within0.5 of a unit from center of light band 306. Controller 122 assesses themisregistration fine adjustment data it receives in order to adjustprinting apparatus 100 operation to compensate, at least partially, forthe misregistration. In an exemplary embodiment of the invention,misregistration within a prescribed limit is considered properlyregistered. For example, if the approximate center of light band 306 iswithin 3.5 units of the “0”, it is considered close enough to beproperly registered in some embodiments of the invention.

In some exemplary embodiments of the invention, calibration print output200 is provided with a plurality of calibration patterns located atvarious points on calibration print output 200. Calibration print output200 depicted in FIG. 2 is shown with 8 calibration patterns, with 4located near the left and right margins, respectively. In an exemplaryembodiment of the invention, each of the plurality of calibrationpatterns is analyzed for the approximate center of the light band as itcorrelates to the offset markers corresponding to that calibrationpattern and the misregistration data is communicated to controller 122.In some embodiments of the invention, at least one calibration patternis printed on the reverse side of the substrate on which calibrationprint output 200 is printed. The reverse side is optionally analyzed ina manner similar to the front in order to correct misregistration indouble sided printing applications. Upon the completion of communicationof the misregistration data to controller 122, calibration print output200 is optionally reprinted (410) to gauge the efficacy of the fineadjustments (408). Misregistration calibration is optionally performed(412) one or more times after completion of the fine adjustments (408)for adjustment verification and/or continued detection and correction ofmisregistration.

In some embodiments of the invention, the fine adjustment pattern isprinted at several places across the image, or all across the image.This allows for the measurement of changes in horizontal registrationacross the page. While measurements taken at a border of the page aregenerally corrected by offsetting one separation from another,variations along the horizontal direction can be corrected by adjustingthe instantaneous timing of the data along the horizontal sweep of thelaser.

In the description and claims of the present application each of theverbs, “comprise” and “include” and conjugates thereof are used toconvey that the object or objects of the verb are not necessarily alisting of all the components, elements or parts of the subject orsubjects of the verb.

While the invention has been described with reference to certainpreferred embodiments, various modifications will be readily apparent toand may be readily accomplished by persons skilled in the art withoutdeparting from the spirit and the scope of the above teachings. Variousembodiments of the invention have been described having specificfeatures. It should be understood that features of the variousembodiments may be combined, where appropriate and features which aredescribed above may be omitted, in some preferred embodiments of theinvention. Therefore, it is understood that the invention may bepracticed other than as specifically described herein without departingfrom the scope of the following claims:

1. A method for detecting and correcting misregistration between aplurality of separations printed by a printing apparatus, comprising:producing a calibration print output on a substrate, the calibrationprint output including at least one calibration pattern, wherein the atleast one calibration pattern translates horizontal misregistration intoa detectable indicator of misregistration in a process direction;detecting misregistration; and, performing a fine adjustment of thehorizontal position of at least one of the separations using the atleast one calibration pattern.
 2. A method according to claim 1, whereina plurality of offset markers are associated with the at least onecalibration pattern.
 3. A method according to claim 2, wherein the fineadjustment is performed using the at least one calibration pattern andthe associated offset markers.
 4. A method according to claim 1, furthercomprising: producing the calibration print output with a plurality ofrough adjustment lines in addition to the at least one calibrationpattern.
 5. A method according to claim 4, further comprising:correcting misregistration by performing a rough adjustment using theplurality of rough adjustment lines prior to performing the fineadjustment.
 6. A method for detecting misregistration between aplurality of separations printed by a printing apparatus, comprising:(a) producing a calibration print output on a substrate, the calibrationprint output including at least one calibration pattern with a pluralityof corresponding offset markers, wherein the at least one calibrationpattern translates horizontal misregistration into a detectableindicator of misregistration in a process direction.
 7. A methodaccording to claim 6, further comprising correcting misregistration byperforming at least one of the following: (b) a rough adjustment using aplurality of rough adjustment lines on the at least one calibrationpattern; and, (c) a fine adjustment using the at least one calibrationpattern and the corresponding offset markers.
 8. A method according toclaim 6, further comprising correcting misregistration by performing:(b) a fine adjustment of the horizontal position of at least one of theseparations using the at least one calibration pattern and thecorresponding offset markers.
 9. A method according to claim 1, whereinthe calibration pattern is produced from a plurality of sub-patterns.10. A method according to claim 9, wherein each of the plurality ofsub-patterns is associated with one of the plurality of separations. 11.A method according to claim 9, wherein at least one of the plurality ofsub-patterns is produced in a different color than the rest of theplurality of sub-patterns.
 12. A method according to claim 1, whereinperforming a fine adjustment includes controlling operation of at leastone laser of the printing apparatus to cause a light band in thecalibration pattern to be centered in relation to the correspondingoffset markers.
 13. A method according to claim 1, wherein thecalibration print output is produced with a calibration pattern on theback of the substrate.
 14. A method according to claim 4, wherein eachof the plurality of rough adjustment lines is associated with one of theplurality of separations.
 15. A method according to claim 4, wherein atleast one of the plurality of rough adjustment lines is produced in adifferent color than the rest of the plurality of rough adjustmentlines.
 16. A method according to claim 4, wherein performing a roughadjustment includes controlling operation of the printing apparatus tocause the plurality of rough adjustment lines to at least touch.
 17. Aprinting apparatus for detection and correction of horizontalmisregistration between a plurality of print separations, comprising: alatent image forming device adapted for forming a plurality of latentimages of said plurality of print separations, developing the latentimage, and printing the latent image on a substrate; a controller incommunication with at least said latent image forming device adapted tocontrol said latent image forming device in the forming of a calibrationpattern, which translates horizontal misregistration into a detectableindicator of misregistration in a process direction, sufficient fordetection and correction of horizontal misregistration between saidplurality of print separations on a calibration print output; and,wherein upon receipt by controller of an indication of horizontalmisregistration, controller controls the latent image forming device toform the plurality of latent images in at least an incremental steptowards correcting the horizontal misregistration.
 18. A printingapparatus according to claim 17, further comprising a user input deviceincluding a display which prompts a user to input detected horizontalmisregistration in communication with said controller.
 19. A printingapparatus according to claim 17, further comprising at least one sensoradapted to sense at least one indicator of horizontal misregistration onsaid calibration print output and communicate the sensed misregistrationto said controller.
 20. A printing apparatus according to claim 19,wherein at least one indicator of horizontal misregistration is acalibration pattern comprised of at least two sub-patterns which whenoverlapped form a light band within the calibration pattern.
 21. Aprinting apparatus according to claim 20, wherein said calibrationpattern includes a plurality of corresponding offset markers.
 22. Aprinting apparatus according to claim 19, wherein the sensor ispositioned to sense a calibration pattern printed on the back of thesubstrate.
 23. A printing apparatus according to any of claim 20 whereinthe sensor is positioned to view more than one calibration patternprinted on the calibration print output.
 24. A printing apparatusaccording to claim 17, wherein at least one of said plurality ofseparations is printed in a different color than the rest of theseparations.
 25. A printing apparatus according to claim 17, wherein atleast one of said plurality of separations is printed in the same coloras another of the separations.